Maintenance grounding device in motor control center with integrated interlock system

ABSTRACT

A motor control center includes an enclosure comprising an isolation switch, a main contactor device, and a ground switch device. The isolation switch is selectively manually operable between a connected state and a disconnected state. In the connected state the isolation switch is adapted to conduct electrical power from an associated power source to the main contactor device and wherein the isolation switch in the disconnected state interrupts conduction of electrical power from the associated power source to the main contactor device. The main contactor device is selectively operable between a conductive state and a non-conductive state, wherein the main contactor device is adapted to electrically connect the isolation switch to the ground switch device and to an associated electrical load when the main contactor device is in its conductive state and wherein the main contactor device disconnects said isolation switch from the ground switch device and the associated electrical load when the main contactor device is in its non-conductive state. The ground switch device is manually operable from an open, ungrounded state in which the main contactor device is electrically disconnected from a ground path to a closed, grounded state in which the main contactor device is electrically connected to the ground path. The motor control center further includes a first interlock device operably connected between the isolation switch and the ground switch device, wherein the first interlock device prevents movement of the isolation switch from the disconnected state to the connected state when the ground switch device is in the grounded state.

BACKGROUND INFORMATION

Motor control centers (MCC) for distributing electrical power toelectric motors are well known in the art. Such motor control centersare connected to a source electrical power, typically three-phase ACpower. A main contactor within the MCC is operable to selectivelyconduct the electrical power to a load such as one or more motors ordisconnect the electrical power from the load. Motor control centersalso include an isolation switch or “isolator” located electricallyupstream relative to the main contactor. The isolator is manuallyoperated by an isolation switch handle. When the isolation switch handleis moved to the open (off) position (typically a “down” position), theisolation switch is opened to disconnect the main contactor and otherdownstream components of the motor control center from the incomingsource of electrical power. When the handle is moved to the closed (on)position (typically an “up” position), the isolation switch is closed toconnect the main contactor and other downstream components to the sourceof electrical power.

During maintenance of such motor control centers, it is desirable formaintenance personnel to ground (earth) the main contactor and othercomponents within the motor control center. A ground switch is providedfor this purpose. The ground switch is selectively operable between aclosed or “grounded” position in which it connects the main contactor toa ground path and an opened or “ungrounded” position in which the groundswitch opens the ground path circuit to disconnect the main contactorfrom the ground path.

It is necessary for safe operation of the motor control center toprevent operation of the ground switch from its opened position to itsclosed position when the isolation switch is in its closed conditionwhich could lead to a dangerous and damaging short circuit condition.For the same reason, it necessary to prevent operation of the isolationswitch from its opened position to its closed position when the groundswitch is already in its closed position. To prevent the condition ofboth the ground switch and the isolator switch from being in aconductive state simultaneously, known motor control centers include akey-based interlock device that uses the same single key for unlocking(allowing operation of) the isolator switch and the ground switch. Thekey is captured in the key interlock device when the interlock is in itsunlocked condition, which prevent a user from closing the isolatorswitch when the ground switch is already closed and vice versa.

While generally safe and effective, these key-based interlock systemsare suboptimal in the sense that a key can be lost, an operator mustperform additional steps, and there is no safety redundancy in the sensethat if the key interlock malfunctions or is otherwise able to beimproperly unlocked, the undesired condition of both the ground switchand isolator switching being in a closed state simultaneously can occur.As such, a need has been identified for a new and improved ground switchinterlock system for a motor control center and other industrialelectrical equipment that overcomes the above deficiencies and othersassociated with known key-based interlock devices while providingsuperior overall results.

BRIEF DESCRIPTION

In accordance with one aspect of the present development, a motorcontrol center includes an enclosure comprising an isolation switch, amain contactor device, and a ground switch device. The isolation switchis selectively manually operable between a connected state and adisconnected state, wherein said isolation switch in said connectedstate is adapted to conduct electrical power from an associated powersource to the main contactor device and wherein the isolation switch inthe disconnected state interrupts conduction of electrical power fromthe associated power source to the main contactor device. The maincontactor device is selectively operable between a conductive state anda non-conductive state, wherein the main contactor device is adapted toelectrically connect the isolation switch to the ground switch deviceand to an associated electrical load when the main contactor device isin its conductive state and wherein the main contactor devicedisconnects said isolation switch from the ground switch device and theassociated electrical load when the main contactor device is in itsnon-conductive state. The ground switch device is manually operable froman open, ungrounded state in which the main contactor is electricallydisconnected from a ground path to a closed, grounded state in which themain contactor is electrically connected to the ground path. The motorcontrol center further includes a first interlock device operablyconnected between the isolation switch and the ground switch device,wherein the first interlock device prevents movement of the isolationswitch from the disconnected state to the connected state when theground switch device is in the grounded state.

In accordance with another aspect of the present disclosure, aninterlock system for electrical equipment includes an isolation switchselectively manually operable between a connected state and adisconnected state, wherein the isolation switch in its connected stateis adapted to conduct electrical power and wherein the isolation switchin its disconnected state interrupts conduction of electrical power. Ahandle is operably connected to the isolation switch and is movablebetween an “off” position corresponding to the disconnected state of theisolation switch and an “on” position corresponding to the connectedstate of the isolation switch. A ground switch device is manuallyoperable from an open, ungrounded state in which the main isolationswitch is electrically disconnected from a ground path to a closed,grounded state in which the isolation switch is electrically connectedto the ground path. An interlock plunger is operably connected to thehandle through an interlock linkage and movable to and between aretracted position corresponding to the “off” position of the handle andan extended position corresponding to the “on” position of the handle.The interlock plunger blocks movement of the ground switch device fromits open, ungrounded state to its closed, grounded state when theinterlock plunger is in its extended position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a motor control center including an integratedinterlock system according to an embodiment of the present invention.

FIG. 2. is an isometric view of the motor control center of FIG. 1.

FIG. 3A is a partial isometric view of the motor control center of FIGS.1 & 2 that shows the isolator ground switch interlock device that ispart of the integrated interlock system, wherein the ground switchdevice is shown in its closed or grounded (conductive) state and theisolator ground switch interlock is shown in its unlocked (disengaged)condition or state.

FIG. 3B is similar to FIG. 3A but shows the isolator ground switchinterlock device from another perspective.

FIG. 3C is an enlarged partial view of a key interlock device that canoptionally form part of the integrated interlock system of the presentdevelopment, with the key interlock device shown in its unlocked(disengaged) state.

FIG. 3D is similar to FIG. 3A but shows the ground switch device in itsopened or ungrounded (non-conductive) state and shows the ground switchinterlock device in its locked (engaged) condition or state.

FIG. 3E is similar to FIG. 3D but shows the isolator ground switchinterlock device from a different perspective (and also shows the keyinterlock device in its locked (engaged) state).

FIG. 3F in an enlarged view of the key interlock device of FIG. 3C inits locked (engaged) state.

FIG. 4 is a partial side view of the motor control center of FIGS. 1 & 2and shows that the integrated interlock system can optionally compriseboth an isolator handle linkage ground switch interlock system and ashared-key ground switch interlock system.

FIG. 5 is a schematic diagram that illustrates the integrated interlocksystem corresponding to FIGS. 1-3F.

FIG. 6 shows a motor control center including the integrated groundswitch interlock system in accordance with an alternative embodiment ofthe present development.

DETAILED DESCRIPTION

FIG. 1. is a front plan view of an or motor control center M or “MCC”with certain panels and components removed for clarity, including anintegrated ground switch interlock system IK according to an embodimentof the present development. FIG. 2 is an isometric view of the motorcontrol center M. With reference to both FIG. 1 and FIG. 2, the motorcontrol center M comprises a cabinet or enclosure E that contains powerswitching and power distribution components PX for powering one or moreloads such as one or more motors or the like. More particularly, the MCCincludes an isolation switch IS (sometimes referred to as an “isolatorswitch” or “isolator”) used to selectively connect and disconnect(isolate) the MCC from a source of electrical power such as three-phaseAC power provided via terminals T (see FIG. 5).

The isolator IS comprises and/or is operably connected to an isolatorhandle IH through which an operator manually controls the operativestate of the isolation switch IS. The isolator handle IH pivots betweenat least a first or “off” position (typically pivoted downward a maximumextent) as shown where the isolator switch IS is opened and disconnectedfrom the incoming power terminals T (a disconnected state) and a secondor “on” position (typically pivoted upward a maximum extent) where theisolator switch IS is closed or connected to the incoming powerterminals T (a connected state). The isolator handle IH is operablymechanically connected to the isolator switch IS through an isolatorswitch linkage ISL that opens and closes the isolator IS in response tomovement of the handle IH between its first and second operativepositions.

The motor control center M further comprises a main contactor device CXthat is electromechanically or otherwise operable to selectively conductelectrical power from the source terminals T to an associated motor orother load (a closed or conductive state) or to disconnect the motor orother load from the electrical power supplied by terminals T (an openedor non-conductive state). The main contactor device CX comprises aplurality of contactors corresponding in number to the number of powerphases (i.e., three contactors X1,X2,X3 for three-phase power)electrically connected (when the isolator IS is closed) on an input sideto the power terminals T and that are connected or adapted to beconnected on an output side or “load side” to the motor(s) or otherload(s) LD (FIG. 5) being powered. When the main contactor CX is in itsconductive state, the contactors X1-X3 are closed and conduct electricalpower from the isolation switch IS to the motor(s) or other load LD.When the main contactor CX is in its non-conductive state, thecontactors X1-X3 are open and interrupt the flow of electrical powerfrom the isolation switch IS to the motor(s) or other load LD. As isgenerally known in the art, a contactor interlock linkage CXL isprovided between the isolator handle and the main contactor device CX toensure at least that the isolator switch IS cannot be opened or closedwhen the main contactor device CX is in its closed (conducting) state.

The motor control center M also comprises a ground switch or groundswitch device GS that is selectively manually operable to connect theload side of the main contactor CX (i.e., the load side of thecontactors X1-X3 of the main contactor CX) to a ground path GP (FIG. 5)to discharge voltage from the motor control center for safety. Theground switch GS is shown separately in FIGS. 3A-3F and comprises atleast one and typically a plurality of ground terminals GT (GT1-GT3),equal in number and respectively electrically connected to the load sideof the contactors X1-X3. The ground switch GS also comprises at leastone and typically a plurality of ground switch contacts GC (GC1,GC2,GC3)equal in number to the ground terminals GT1-GT3. When the ground switchGS is closed (a “grounded state” as shown in FIGS. 3A & 3B), the groundswitch contacts GC1-GC3 contact and electrically connect respectivelywith the ground terminals GT1-GT3 to connect the ground terminalsGT1-GT3 (and the associated contactor X1-X3 to which they arerespectively connected) to the ground path GP. When the ground switch GSis opened (an “ungrounded state” as shown in FIGS. 3D & 3E), the groundswitch contacts GC1-GC3 separate respectively from the ground terminalsGT1-GT3 to disconnect the ground terminals GT1-GT3 (and the associatedcontactor X1-X3 to which they are respectively connected) from theground path GP.

With continuing reference particularly to FIGS. 3A-3F, it can be seenthat the ground switch GS comprises a rotatable ground shaft GSX that isconnected to the ground path GP. The ground switch contacts GC1-GC3 areconnected to and rotate with the ground shaft GSX. Rotation of theground shaft GSX in a first angular direction moves the ground switchcontacts GC1-GC3 into contact with the respective ground terminalsGT1-GT3 for connection to and completion of the ground path GP. Rotationof the ground shaft GSX in an opposite, second angular directionseparates the ground switch contacts GC1-GC3 from the respective groundterminals GT1-GT3 to open or interrupt the ground path GP.

The ground shaft GSX is manually rotatable using a wrench or other toolTL that is removably connected to a rotatable torque input head HX. Thetorque input head HX is connected to and rotates with an input gear GR1that is drivingly engaged with the ground shaft GSX such that manuallyrotation of the torque input head HX in first and second directionscorrespondingly rotates the input gear GR1 and also the ground shaft GSXin first and second angular directions. As shown herein the input gearGR1 is non-rotatably connected to an input shaft GR1X that is rotatablysupported relative to the enclosure E, and the torque input head HX isnon-rotatably connected to and/or forms a part of the input shaft GR1X.In the illustrated embodiment, a main gear GR2 such as a bevel gear isnon-rotatably connected to the ground shaft GSX and is engaged with theinput gear GR1 which acts as a pinion gear to drive the main gear GR2and ground shaft GSX in the first and second angular directions to openand close the ground contacts GC1-GC3. The ground switch device GSfurther comprises an output gear or driven gear GR3 that is drivinglyengaged with the main gear GR2 and that rotates in first and secondopposite directions in response to rotation of the main gear GR2. Inparticular, the driven gear GR3 rotates in opposite directions ascompared to the input gear GR1 such that the driven gear GR3 rotates insecond and first opposite directions when the input gear GR1 rotates infirst and second directions, respectively. The gears GR1-GR3 define aground switch device gear train.

The ground switch GS comprises at least one and optionally first andsecond interlock devices for preventing closing of the ground switchcontacts GC1-GC3 when the isolator switch IS is closed and forpreventing closing of the isolator switch IS when the ground switchcontacts GC1-GC3 are closed. In particular, the ground switch comprisesa first interlock device IX1 (sometimes referred to herein as the“isolator interlock device”) and optionally also includes a secondinterlock device IX2 (sometimes referred to herein as the “key interlockdevice”).

The first (isolator) interlock device IX1 comprises a first lock base orlock block B1 that is non-rotatably connected to the driven output gearGR3. As shown, the driven output gear GR3 is non-rotatably connected toan output shaft GR3X that is rotatably supported relative to theenclosure E, and the first lock block B1 is non-rotatably connected tothe output shaft GR3X so as to rotate in unison with the output gearGR3. The first lock block B1 can be non-circular and comprise aplurality of flat surfaces. The first lock block B1 comprises a firstlock recess RC1.

The first interlock device IX1 further comprises an interlock membersuch as an interlock member or interlock plunger LP1 that is connectedto an isolation switch interlock linkage IXL that is operativelyconnected to the isolator switch handle IH. Due to the interlock linkageIXL, movement of the isolator switch handle IH from the first (off)position to the second (on) position induces movement of the interlockplunger LP1 in a first direction D1 toward the lock block B1 (downwardin a typical installation) to a deployed or extended position as shownin FIGS. 3D & 3E. Conversely, the interlock linkage IXL causes theinterlock plunger LP1 to move in a second direction D2, opposite thefirst direction D1, away from the lock block B1 (upward in a typicallyinstallation) to a stored or retracted position when the isolator switchhandle IH is moved from its second (on) position toward its first (off)position as shown in FIG. 3A.

The first lock block B1 is conformed and dimensioned to block andprevent movement of the interlock plunger LP1 in the first direction D1for all angular positions of the lock block B1 except when the lockrecess RC1 is located in a select plunger receiving position where thelock recess RC1 is registered or aligned with the interlock plunger LP1.When the lock plunger LP1 is located in the first lock recess RC1, theisolator interlock device IX1 is in its “engaged” position where itprevents operation of the ground switch device GS and when the lockplunger LP1 is retracted and withdrawn from the lock recess RC1 theisolator interlock device IX1 is in its “disengaged” position where itallows operation of the ground switch device GS.

When the isolator interlock is disengaged and moved toward its engagedposition, contact between the interlock plunger LP1 and the first lockblock B1 blocks movement of the lock plunger LP1 in the first directionD1 which, in turn, blocks movement of the isolator handle to its “on”(conducting) position except when the lock recess RC1 is aligned withthe lock plunger LP1. The first lock block B1 is keyed in a selectangular position on the output shaft GR3X such that the lock recess RC1is only located in the lock plunger receiving position where it isaligned with and open to receive the interlock plunger LP1 when theground switch GS is located or arranged in its opened (non-conducting)position, i.e., when the ground contacts GC1-GC3 are open. Thus, whenthe lock recess RC1 is aligned with the interlock plunger LP1, theinterlock plunger LP1 is able to move in the direction D1 into the lockrecess RC1 (to engage the isolator interlock device IX1) which allowsthe isolator handle IH to move to its “on” (conducting) position.Furthermore, those of ordinary skill in the art will recognize that thelock recess RC1 is conformed and dimensioned to receive the interlockplunger LP1 therein in a manner such that, when the isolator interlocksystem IX1 is engaged, the interlock plunger LP1 abuts and preventsrotation of the lock block B1 and, consequently, also prevents rotationof the output gear GR3, main gear GR2, and ground shaft GSX when theinterlock plunger LP1 is received and seated in the lock recess RC1. Inthis manner, the ground switch is restrained in its opened(non-conducting) state when the isolator handle IH is located in itssecond (on) position corresponding the closed (conducting) state of theisolator switch IS to prevent closing of the ground contacts GC1-GC3when the isolator switch is in a conductive state. The interlock plungerLP1 is shown as being moved linearly in the first and second oppositedirections D1,D2 by the interlock linkage IXL, but those of ordinaryskill in the art will recognize that the interlock plunger canalternatively move along a non-linear path between its retracted anddeployed positions.

The second (key) interlock device IX2 similarly comprises a second lockmember or second lock block B2 that is non-rotatably connected to theinput gear GR1. As shown, the input gear GR1 is non-rotatably connectedto the input shaft GR1X that is rotatably supported relative to theenclosure E, and the second lock block B2 is non-rotatably connected tothe input shaft GR1X so as to be non-rotatably coupled to and rotate inunison with the input gear GR1. The second lock block B2 can benon-circular and comprise a plurality of flat surfaces. The second lockblock B2 comprises a second lock recess RC2 (see FIGS. 3E & 3F).

The key interlock device IX2 further comprises an interlock member suchas a keylock plunger or other lock member LP2 that is connected to akey-lock assembly LA that is operated with a removable key K. The lockassembly LA is secured to the enclosure E. When the key K is operablyengaged in the lock assembly LA, rotational movement of the key K in afirst direction from a first (unlocked) position to a second (locked)position induces movement of the lock member LP2 in an extensiondirection Z1 (FIG. 3C) toward the lock block B2 (downward in theillustrated embodiment) to a deployed or extended or “locked” positionas shown in FIGS. 3E & 3F. Conversely, when the key K is rotated in asecond opposite direction from the second (locked) position to the first(unlocked) position, the lock assembly LA induces movement of the lockmember LP2 in a retraction direction Z2 (FIG. 3C), opposite theextension direction Z1, away from the lock block B2 (upward in theillustrated embodiment) to a retracted or “unlocked” position as shownin FIGS. 3A-3C. The second lock block B2 is conformed and dimensioned tobe contacted by and to block and prevent movement of the lock member LP2in the extension direction X1 direction Z1 into its extended positionfor all angular positions of the second lock block B2 except when thesecond lock recess RC2 thereof is located in a select plunger receivingposition where the recess RC2 is registered or aligned with the lockmember LP2. The second lock block B2 is keyed in a select angularposition on the input shaft GR1X such that the second lock recess RC2 isonly located in its receiving position where it is aligned with and opento receive the lock member LP2 when the ground switch GS in in itsopened (non-conducting) position, i.e., when the ground contacts GC1-GC3are open as shown in FIGS. 3E & 3F Furthermore, those of ordinary skillin the art will recognize that the lock recess RC2 is conformed anddimensioned to receive the lock member LP2 therein in a manner such thatthe lock member LP2 abuts and prevents rotation of the second lock blockB2 and, consequently, also prevents rotation of the input gear GR1, maingear GR2, and ground shaft GSX when the lock member LP2 is received andseated in the lock recess RC2. In this manner, the ground switch GS isrestrained in its opened (non-conducting) state when the lock assemblyLA is positioned in its locked state. The lock member LP2 is shown asbeing moved linearly in the first and second opposite directions Z1,Z2by the lock assembly LA, but those of ordinary skill in the art willrecognize that the lock member LP2 can alternatively move along anon-linear path between its retracted and extended positions.

The key K is trapped or captured in the lock assembly LA while the lockassembly LA is in its unlocked condition and can only be removed fromthe lock assembly LA when the lock assembly LA is in its lockedcondition. This allows the key K to be used in a shared-key interlocksystem where the same key K is used to operate first and second keyinterlock devices to ensure that only one of the key interlock devicesis unlocked at any given time. FIG. 4 shows that the motor controlcenter M can include the key interlock device IX2 as described above andalso includes an isolator handle key interlock device HX including ahandle lock assembly HLA that selectively locks the isolator handle IHin its “off” position when locked to prevent movement of the isolatorhandle IH to the “on” position when locked. The handle key interlockdevice HX uses the same single K as the ground switch key interlockdevice IX2 and the key K is captured in the handle interlock device HXwhen it is unlocked to ensure that the isolator handle IH is moved toand locked in its “off” position before the key K can be removed fromthe handle interlock device HX and inserted into the ground switch keyinterlock device IX2 to unlock the ground switch device GS to allow theground switch device to be moved to its conductive or “on” state.

Another advantage of a ground switch interlock system IK provided inaccordance with an embodiment of the present development is that, unlikeknown systems, the ground switch device GS is located on the load-sideof the main contactor CX, i.e., electrically between the motor or otherassociated load LD being powered by the motor control center M and thecontactors X1-X3 of the main contactor CX. As shown in FIG. 5, the motorcontrol center M is located electrically between the incoming source ofelectrical power P and the motor or other load LD. The isolation switchIS is selectively operable to connect or disconnect the main contactorCX from the electrical power source P. The ground switch interlockdevice IX1 operably interconnects the isolation switch IS and the groundswitch device GS via interlock linkage IXL as described above. Theisolator switch IS is electrically connected to the main contactor CX,and the main contactor CX is electrically connected on its downstream oroutput side to the load LD. The ground switch device GS is also locatedon the downstream or output side of the main contactor CX such that theground switch contacts GC (GC1-GC3) are electrically located between themain contactor CX and the load LD. When the ground switch contacts GC ofthe ground switch device GS are closed, the ground path GP is connectedbetween the load LD and the main contactor CX to safely ground or earththe main contactor CX and the load LD. This arrangement ensures that theload side of the main contactor CX is grounded when the ground switchdevice GS is in its closed (conductive) state.

FIG. 1 illustrates another main advantage of an embodiment of thepresent development. There, it can be seen that the ground switchcontacts GC1-GC32 are vertically aligned respectively with the groundterminals GT1-GT3. In the case of three-phase AC power, the three groundterminals GT1-GT3 respectively associated with the three-phases ofelectrical power are horizontally spaced-apart from each other in a row.Likewise, the three ground switch contacts GC1-GC3 are horizontallyspaced-apart from each other in a row and are vertically alignedrespectively with the ground terminals GT1-GT3 such that each groundterminal GT1-GT3 lies in a common vertical reference plane with respectto its associated ground switch contact GC1-GC3 for all operativepositions of the ground switch contact GC1-GC3. Thus, the threeassociated ground terminal/ground contact pairs GT1/GC1, GT2/GC2, andGT3/GC3 respectively lie in three parallel, spaced-apart verticalreference planes. This arrangement provides a simple and effectivemethod for a human operator to quickly and safely assess the groundstate (grounded or ungrounded) of each of the ground terminals GT1-GT3by visual inspection in that the operator will be able to determinevisually whether each ground terminal GT1-GT3 is engaged by itsassociated ground contact GC1-GC3 or not by viewing the ground switchdevice GS from the front as shown in FIG. 1, such as through an accessopening or window defined in the enclosure E. The first, second, andthird ground terminals GT1-GT3 and also the first, second, and thirdground contacts GC1-GC3 are vertically aligned with and are thusvisually associated with both the first, second, and third contactorsX1-X3 and also with the first, second, and third power phases,respectively.

The physical position of the ground switch device GS relative to themain contactor CX can be altered such as by locating the ground switchdevice GS vertically above the main contactor CS as shown for the motorcontrol center M′ in FIG. 6 rather than vertically below the maincontactor CX as shown in FIG. 1 (or the ground switch device GS can belocated laterally adjacent the main contactor CX). In all cases,however, the ground switch device GS is electrically connected on theload side or output side of the main contactor CX as described above.

In the preceding specification, various embodiments have been describedwith reference to the accompanying drawings. It will, however, beevident that various modifications and changes may be made thereto, andadditional embodiments may be implemented, without departing from thebroader scope of the invention as set forth in the claims that follow.The specification and drawings are accordingly to be regarded in anillustrative rather than restrictive sense.

The following is claimed:
 1. A motor control center comprising: anenclosure comprising an isolation switch, a main contactor device, and aground switch device; said isolation switch selectively manuallyoperable between a connected state and a disconnected state, whereinsaid isolation switch in said connected state is adapted to conductelectrical power from an associated power source to said main contactordevice and wherein said isolation switch in said disconnected stateinterrupts conduction of said electrical power from the associated powersource to the main contactor device; said main contactor deviceselectively operable between a conductive state and a non-conductivestate, wherein said main contactor device is adapted to electricallyconnect said isolation switch to said ground switch device and to anassociated electrical load when said main contactor device is in aconductive state and wherein said main contactor device disconnects saidisolation switch from the ground switch device and the associatedelectrical load when said main contactor device is in a non-conductivestate; said ground switch device manually operable from an open,ungrounded state in which said main contactor device is electricallydisconnected from a ground path to a closed, grounded state in whichsaid main contactor device is electrically connected to said groundpath; and said ground switch device comprising: an isolator interlockdevice operably connected between said isolation switch and said groundswitch device, wherein said isolator interlock device prevents movementof said isolation switch from said disconnected state to said connectedstate when said ground switch device is in said grounded state, and, akey interlock device configured to lock said ground switch device insaid open, ungrounded state so that movement of said ground switchdevice to said grounded state is blocked.
 2. The motor control center asset forth in claim 1, further comprising an isolation switch handleoperably connected to said isolation switch and movable between an “off”position corresponding to said disconnected state of said isolationswitch and an “on” position corresponding to said connected state ofsaid isolation switch, wherein said isolator interlock device blocksmovement of said isolation switch handle from said “off” position tosaid “on” position when said ground switch device is positioned in saidgrounded state.
 3. The motor control center as set forth in claim 2,wherein said isolator interlock device prevents movement of said groundswitch device from said ungrounded state to said grounded state whensaid isolation switch handle is located in said “on” position.
 4. Themotor control center as set forth in claim 3, wherein said isolationswitch handle is operably connected to said isolation switch by amechanical isolator switch linkage and wherein said isolator interlockdevice comprises a lock plunger operably connected to said isolationswitch handle by an isolation switch interlock linkage.
 5. The motorcontrol center as set forth in claim 4, wherein: said isolation switchinterlock linkage moves said lock plunger to a retracted position whensaid isolation switch handle is located in said “off” position; saidisolation switch interlock linkage moves said lock plunger to anextended position when said isolation switch handle is located in said“on” position; said ground switch device comprises at least one movableground contact that moves between an opened position in said ungroundedstate to a closed position in said grounded state; and, said lockplunger blocks movement of said at least one movable ground contact fromsaid opened position to said closed position when said lock plunger isin said extended position.
 6. The motor control center as set forth inclaim 5, wherein said isolator interlock device comprises a first lockblock that is contacted by said lock plunger and blocks movement of saidlock plunger to said extended position when said lock plunger moves fromsaid retracted position toward said extended position when said at leastone movable ground contact is in said closed position.
 7. The motorcontrol center as set forth in claim 6, wherein said first lock blockblocks movement of at least one movable ground contact from said openedposition to said closed position when said lock plunger is in saidextended position.
 8. The motor control center as set forth in claim 7,wherein said first lock block comprises a first lock recess located insaid first lock block, wherein said lock plunger is seated in said firstlock recess when said lock plunger is in said extended position and saidat least one movable ground contact is in said closed position.
 9. Themotor control central as set forth in claim 8, wherein said at least onemovable ground contact is connected to a rotatable ground shaft that iselectrically connected to said ground path, said ground switch devicecomprising a main gear connected to said ground shaft and an output gearengaged with said main gear; wherein said first lock block isnon-rotatably coupled with said output gear such that rotation of saidground shaft alters an angular position of said first lock block; and,wherein said first lock block is located in a select angular position inwhich said first lock recess is aligned with said lock plunger when saidground shaft is rotated to a position corresponding to said closedposition of said at least one movable ground contact.
 10. The motorcontrol center as set forth in claim 9, further comprising an input gearengaged with said main gear, wherein said input gear is non-rotatablyconnected to a torque input head that is selectively manually rotatableto rotate said input gear, said main gear, and said output gear.
 11. Themotor control center as set forth in claim 10, wherein the key interlockdevice comprises a first key interlock device operable by a key betweena locked state and an unlocked state, wherein said first key interlockdevice blocks rotation of said input gear when said first key interlockdevice is in said locked state.
 12. The motor control center as setforth in claim 11, wherein the key interlock device further comprises asecond key interlock device operable by said key between a locked stateand an unlocked state, wherein said second key interlock deviceselectively blocks movement of said isolation switch handle when saidsecond key interlock device is in said locked state, wherein: said keythat operates said first and second key interlocks is a single, commonkey; when one of the first and second key interlock devices is in saidrespective locked state, another one of the first and second keyinterlock devices is in in said respective unlocked state; and, said keyis captured in said another one of said first and second key interlockdevices that is in said respective unlocked state such that said anotherone of said first and second key interlock devices must be moved to saidrespective locked state before said key is removed for use in movingsaid one of said first and second key interlock devices from saidrespective locked state to said respective unlocked state.
 13. The motorcontrol center as set forth in claim 12, wherein the first key interlockdevice further comprises a second lock block non-rotatably coupled tosaid input gear, and a lock member that engages and prevents rotation ofsaid second lock block when said lock member is in an extended positionand said ground switch device is in said open, ungrounded state.
 14. Themotor control center as set forth in claim 5, wherein said ground switchdevice is located on a load side of said main contactor device.
 15. Themotor control center as set forth in claim 14, wherein said groundswitch device further comprises: first, second, and third groundterminals electrically connected to respective first, second, and thirdelectrical phases of said main contactor device; said at least onemoveable ground contact comprises first, second, and third groundcontacts that respectively contact said first, second, and third groundterminals when said first, second, and third ground contacts are in saidrespective closed positions.
 16. The motor control center as set forthin claim 15, wherein said first, second, and third ground terminals arerespectively vertically aligned with said first, second, and thirdground contacts.
 17. The motor control center as set forth in claim 16,wherein: said ground switch device is located vertically above orvertically below said main contactor device; said main contactor devicecomprises first, second, and third contactors electrically connectedrespectively to said first, second, and third ground terminals; and,said first, second, and third ground terminals are horizontallyspaced-apart from each other in a row so that said first, second, andthird ground terminals are visually associated with said first, second,and third contactors, respectively.
 18. A motor control centercomprising: an enclosure comprising an isolation switch, a maincontactor device, and a ground switch device; said isolation switchselectively manually operable between a connected state and adisconnected state, wherein said isolation switch in said connectedstate is adapted to conduct electrical power from an associated powersource to said main contactor device and wherein said isolation switchin said disconnected state interrupts conduction of said electricalpower from the associated power source to the main contactor device; anisolation switch handle operably connected to said isolation switch by amechanical isolator switch linkage and movable between an “off” positioncorresponding to said disconnected state of said isolation switch and an“on” position corresponding to said connected state of said isolationswitch; said main contactor device selectively operable between aconductive state and a non-conductive state, wherein said main contactordevice is adapted to electrically connect said isolation switch to saidground switch device and to an associated electrical load when said maincontactor device is in a conductive state and wherein said maincontactor device disconnects said isolation switch from the groundswitch device and the associated electrical load when said maincontactor device is in a non-conductive state; said ground switch devicemanually operable from an open, ungrounded state in which said maincontactor device is electrically disconnected from a ground path to aclosed, grounded state in which said main contactor device iselectrically connected to said ground path; said ground switch devicecomprising an isolator interlock device operably connected between saidisolation switch and said ground switch device, wherein: said isolatorinterlock device: (i) prevents movement of said isolation switch handlefrom said “off” position to said “on” position and prevents movement ofsaid isolation switch from said disconnected state to said connectedstate when said ground switch device is in said grounded state; and (ii)prevents movement of said ground switch device from said ungroundedstate to said grounded state when said isolation switch handle islocated in said “on” position; wherein said isolator interlock devicecomprises a lock plunger operably connected to said isolation switchhandle by an isolation switch interlock linkage, and wherein: (i) saidisolation switch interlock linkage moves said lock plunger to aretracted position when said isolation switch handle is located in said“off” position; (ii) said isolation switch interlock linkage moves saidlock plunger to an extended position when said isolation switch handleis located in said “on” position; (iii) said ground switch devicecomprises at least one movable ground contact that moves between anopened position in said ungrounded state to a closed position in saidgrounded state; and, (iv) said lock plunger blocks movement of said atleast one movable ground contact from said opened position to saidclosed position when said lock plunger is in said extended position. 19.The motor control center as set forth in claim 18, wherein said isolatorinterlock device comprises a first lock block that is contacted by saidlock plunger and blocks movement of said lock plunger to said extendedposition when said lock plunger moves from said retracted positiontoward said extended position when said at least one movable groundcontact is in said closed position.
 20. The motor control center as setforth in claim 19, wherein said first lock block blocks movement of atleast one movable ground contact from said opened position to saidclosed position when said lock plunger is in said extended position.